1. Field of the Invention
This invention relates to a semiconductor device, or in particular to a semiconductor device having a partial trench isolation structure.
2. Description of the Background Art
A semiconductor device of a SOI (silicon on insulator) structure (hereinafter referred to as the SOI device) in which a buried oxide film and a SOI layer are formed on a silicon substrate finds applications as portable devices due to the feature that the parasitic capacitance can be reduced and the operation is fast and stable with a low power consumption.
An example of the SOI device has a full trench isolation (FTI) structure in which a trench reaching a buried oxide film is formed in the surface of a SOI layer and an insulating material is buried in the trench. The full trench isolation insulating film thus formed is used to isolate elements electrically.
Various problems are caused, however, by the substrate floating effect in which carriers (holes for NMOS) generated by the impact ionization stay in the body region including the channel forming region, with the result that a kink is generated, the operation withstanding voltage is reduced or the unstable potential of the body region causes the frequency dependence of the delay time.
In view of this, the partial trench isolation (PTI) structure has been conceived, as disclosed in Yuuichi Hirano et al., “Bulk-Layout-Compatible 0.18-μm SOI-CMOS Technology Using Body-Tied Partial-Trench-Isolation (PTI)”, “IEEE TRANSACTION ON ELECTRON DEVICES. vol. 48, No. 12, DECEMBER 2001, pp. 2816-2822”: Non-Patent Document 1), in which a trench is formed in the surface of the SOI layer in such a manner as to leave a SOI layer of a predetermined thickness between the trench bottom portion and the buried oxide film, and which has a partial trench isolation insulating film formed by burying an insulating material in the trench.
By employing the PTI structure, carriers can be moved through the well region under the partial trench isolation insulating film, the carriers are prevented from staying in the body region, and the potential of the body region can be fixed through the well region. Thus, the various problems which otherwise might be caused by the substrate floating effect are not posed.
In the case where the PTI structure is employed, a high-concentration impurities region of the same conduction type as the body region is formed as a body-tied region in the surface of the SOI layer outside the ends of the gate electrode along the gate width, and this body-tied region is electrically connected to an overlying wiring layer thereby to fix the potential of the body region.
In the semiconductor device not using the PTI structure, on the other hand, the use of the T-shaped gate electrode or the source-tied gate electrode as shown in Kerry Bernstein and Norman J. Rohrer, “SOI CIRCUIT DESIGN CONCEPTS”, Kluwer Academic Publishers, pp. 22-23 (Non-Patent Document 2)” has been proposed to fix the potential of the body region.
Specifically, FIG. 2.8(a) of Non-Patent Document 2 shows a configuration in which the gate electrode is in the shape of T and the portion corresponding to the leg of T functions substantially as a gate while the portion corresponding to the head of T extends to completely cover the short sides of the source region and the drain region with a body contact region formed outside the head of T. The body contact region contains high-concentration impurities of the same conduction type as the body region.
Also, FIG. 2.8(b) of Non-Patent Document 2 shows the gate electrode of what is called the source-tied type in which a protrusion is formed toward the source region from the neighborhood of the central portion along the gate width of the gate electrode, and a high-concentration impurities region having a different conduction type from the source region is formed in the surface of the source region under the protrusion.
With the size reduction of the semiconductor device, the gate length is also shortened. In the T-shaped gate electrode, however, the potential is fixed at one end of the gate electrode, and therefore, the gate length is shortened. In the case where the gate width is very large, in contrast, the resistance value of the body region is increased and the potential of the body region cannot be sufficiently fixed, thereby giving rise to the likelihood of generating a kink or reducing the operation withstanding voltage.
Also, in the case of the T-shaped gate electrode, the lower part of the head of T constitutes the same impurities region as the body region. Since a gate insulating film exists between the impurities region and the head of T, however, the unrequired capacitance component exists there and may affect the operation of the transistor.
In the source-tied gate electrode having a very long gate width as compared with the gate length, on the other hand, an increased number of protrusions toward the source region is equivalent to the division of the gate electrode into a plurality of parts, thereby making it possible to fix the potential of the body region for each of the short gate electrodes.
In forming a high-concentration impurities region in the surface of the source region under the protrusion, however, impurities are introduced by ion implantation. Due to the displacement attributable to the precision of the implantation mask, therefore, the position of the high-concentration impurities region under the protrusion and the potential fixed position are varied, thereby substantially resulting in the variation of the length of each of a plurality of divided gate electrodes. This may cause the variation of the operation characteristics of the transistor.
As explained above, the gate length is shortened with the size reduction of the semiconductor device, and in the case where the gate width is very large as compared with the gate length, the potential of the body region fails to be sufficiently fixed, resulting in the likelihood of a kink or reduction in the operation withstanding voltage. This problem cannot be obviated by the T-shaped gate electrode or the source-tied electrode.